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Wednesday, 1 January 2014

Lead Corrosion in Acids

Lead
forms a protective film, which if undisturbed preserves the metal
below this layer.

The
corrosion resistance of lead is based on its ability to readily form
a tenacious coating of a reaction product. This then becomes a
protective coating. Protective coatings on lead may form as the
result of exposure to sulphates, oxides, carbonates, chromates, or
chemical complexes.

Handbook
of Corrosion Data, by Bruce D Craig, p26

Lead
is resistant to corrosion especially “with solutions containing
sulphate ions, such as sulphuric acid.”

However, the new or bright metal reacts quickly with a variety of alkalis and many organic (although not most inorganic) acids. “...Lead is not stable in nitric and acetic acids, nor in
alkalis. The metal does not resist nitric acid. Lead corrodes rapidly
in acetic and formic acids.” (Handbook
of Corrosion Data, by Bruce D Craig, p.29)

Lead
has very limited resistance to acetic acid.... Dilute [acetic acid],
even at room temperature attacks lead at rates exceeding 1.3mm/year.
These rates increase rapidly with increasing aeration and velocity
However … acetic acid … has little effect at strengths of 52% to
70%.

The
corrosion rate in acid increases rapidly in the presence of oxygen
and also in oxygen in combination with soft waters such as rain and
distilled water. Corrosion increases at the rate approximately
proportional to the oxygen content of the water.”

Handbook
of Corrosion Data, by Bruce D Craig, p.26, 29

This another good reason to avoid vinegar as a cleaning agent for leaded windows.

Lead
dissolves in organic acids (in the presence of oxygen). Lead also
dissolves in quite concentrated alkalis
(≥10%) because of the characteristic of the lead salts that can act
as either an acid or an alkali. These salts are soluble in the
presence of water and oxygen.

lye
(generic term, for either of the previous two, or even for a
mixture)

Calcium
hydroxide (saturated solution known as "limewater")

Magnesium
hydroxide is an example of an atypical alkali since it has low
solubility in water (although the dissolved portion is considered a
strong base due to complete dissociation of its ions).

Although this has been a rather technical posting, these
data show that lead is subject to rapid attack by both organic (and
some inorganic) acids and alkalis in relatively low concentrations
when in the presence of aerated water. However in normal
environmental conditions the protective reaction layer avoids much of
this vulnerability.

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